1. Field of the Invention
This invention relates to a discharge lamp lighting apparatus used for vehicles.
2. Description of the Prior Art
Heretofore, a lighting apparatus such as that shown in FIG. 34 is known (see Japanese Patent Application Early Laid-open Publication No. Hei 3-8299).
Referring to FIG. 34, the numeral 1 denotes a DC voltage raising circuit for raising the DC voltage of a battery 2, the numeral 3 denotes an HF (high frequency) voltage raising circuit for converting the output voltage of the DC voltage raising circuit 1 into a sinusoidal AC voltage, the numeral 4 denotes an ignitor circuit for applying a high voltage to a discharge lamp 5, the numeral 6 denotes an ignitor starting circuit for outputting a starting signal to the ignitor circuit 4, and the numeral 7 denotes a control circuit for controlling the output voltage of the DC voltage raising circuit 1.
The control circuit 7 is made up of a voltage calculating portion 7a for detecting the output voltage of the DC voltage raising circuit 1, a current calculating portion 7b for detecting the output current of the DC voltage raising circuit 1, and a pulse duration modulating portion 7c. The pulse duration modulating portion 7c serves to generate a rectangular pulse having a duty cycle corresponding to each of signals output by the voltage calculating portion 7a and the current calculating portion 7b. The numeral 8 denotes a connector.
When a lighting switch S is turned on, a high voltage trigger is generated in the ignitor circuit 4 by means of the ignitor starting circuit 6. This high voltage trigger causes the discharge lamp 5 to be lighted, and then the output voltage of the DC voltage raising circuit 1 is controlled at any time by the control circuit 7. This leads to a stationary state of the discharge lamp 5.
However, since the conventional discharge lamp lighting apparatus has no feedback control means for suitably setting electric power to be supplied to a load (i.e., the discharge lamp 5), it takes much time to cause the discharge lamp 5 to reach a normal discharging state after turning on the switch S. That is, the conventional apparatus has a fault in that a rise time is long.
Further, the conventional apparatus has a fault as is clear from the following. When the discharge lamp 5 is lighted, a timer (not shown) is actuated. After the lapse of a predetermined time, the timer stops the ignitor starting circuit 6. This arrangement prevents the high voltage trigger from applying a voltage to the lamp 5 or connector 8 for more than a given time. Therefore, if the connection of the connector 8 is temporarily cut by, e.g., vibrations, the discharge of the lamp 5 is attenuated and accordingly the lamp 5 will not be again lighted by the voltage output by the HF voltage raising circuit 3. Therefore, the lamp 5 is extinguished temporarily or completely.
Further, the conventional apparatus has another fault as is clear from the following. The HF voltage raising circuit 3 unconditionally outputs an AC voltage as a lamp voltage regardless of a drop of the source voltage of the apparatus. Therefore, for example, if the HF voltage raising circuit 3 outputs the AC voltage by the switching of four transistors when the output voltage of the DC voltage raising circuit 1 drops and then the lamp voltage drops to a discharge maintaining voltage, the lamp 5 is easily extinguished owing to a transient voltage drop caused in the switching.